GaN, AlN, and InN, etc., which are nitride semiconductors, or materials made of mixed crystals of these nitride semiconductors, have a wide band gap, and are used as high output electronic devices or short-wavelength light emitting devices. Among these, as high output devices, technologies are developed in relation to field effect transistors (FET), more particularly, High Electron Mobility Transistors (HEMT) (see, for example, Patent Document 1). A HEMT using such a nitride semiconductor is used for high output/high efficiency amplifiers and high power switching devices.
As a field effect transistor using a nitride semiconductor, there is a HEMT in which GaN is used as an electron transport layer and AlGaN is used as an electron supply layer. In the electron transport layer, 2DEG (Two-Dimensional Electron Gas) is generated, by the functions of piezoelectric polarization and spontaneous polarization in GaN. Furthermore, in order to respond to demand for high output and high efficiency of the HEMT, there is a HEMT in which GaN is used in the electron transport layer and InAlN is used in the electron supply layer. Because InAlN has high spontaneous polarization, by using InAlN in the electron supply layer, a high concentration of 2DEG is induced, and it is possible to cause more drain currents to flow in the electron supply layer, compared to the case of a HEMT using AlGaN.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2002-359256    Patent Document 2: Japanese Laid-Open Patent Publication No. 2010-45343    Patent Document 3: Japanese Laid-Open Patent Publication No. 2013-251544    Patent Document 4: Japanese Laid-Open Patent Publication No. 2013-222800    Patent Document 5: Japanese Laid-Open Patent Publication No. 2013-235986